Watermarking involves embedding information in data so that the information is hidden in the normal use of that data. Ideally, the information is embedded so that 1) it does not significantly reduce data quality, 2) the information can be recovered and 3) it is robust both to malicious attacks and nonmalicious attacks such as typical processing operations that may be performed on the data. Thus, watermarking involves a trade-off among data quality, robustness and capacity of information carried by the watermark.
Nonmalicious attacks include a variety of operations performed on the data that have a side effect of weakening the strength of the watermark signal with respect to the the data. For example, if the data is image data, that data may be, for example, compressed, resized, cropped, converted between analog and digital format, or sub sampled.
Malicious attacks include a variety of attacks designed to weaken the strength of the signal relating to the watermark embedded in the data, which in turn reduces the ability to detect the watermark. For example, a common malicious attack on several identical images with different watermarks is to average them together to produce a new, identical image with the strength of the watermarks significantly reduced. Some operations may be considered both malicious and nonmalicious attacks.
For example, a common activity in the piracy of motion pictures is capturing a movie displayed in a movie theatre using a camcorder. The movie then is transferred to a computer and distributed. Thus, any watermark embedded in an image of the motion picture is subjected to several operations that affect the watermark: conversion to film, projection on a screen, capture on a camcorder, and probably digitizing and compression. All of these operations have the effect of reducing the strength of the watermark in the motion picture and thus are kinds of attacks.